This application claims the priority of Japanese Patent Application NO.6-152701 (152701/1994) filed Jun. 9, 1994 which is incorporated herein by reference.
Diamond as a natural resource has suffered from a poor yield. Natural diamond crystals are small. If natural diamond crystals were sliced and polished, no wide wafers would be obtained. There is no probability of producing wide diamond wafers from natural diamond crystals. Bulk diamond crystals can be synthesized by an ultra-high pressure method which makes a diamond crystal from solid carbon under an ultra-high pressure at a high temperature. But such synthesized diamond is small granules. The ultra-high pressure method cannot produce big bulk diamond crystals. The ultra-high pressure diamond is a lump. The lump of diamond should be polished in order to make a thin disc (wafer). The polishing of diamond is, however, very difficult, since diamond is the hardest material. Polishing a diamond lump into a thin diamond plate Is nearly impossible from the standpoint of cost, because the polishing will enormously consume time and polishing medium. If a bulk diamond could be polished, a thin plate would be too narrow to make a plenty of devices on the diamond plate. Industrial applications of diamond are subject to wide wafers.
Another synthesis of diamond is CVD methods. The CVD methods can make a diamond film on a suitable substrate from a vapor phase instead of bulk, lump crystals. The CVD methods supply hydrogen gas and a hydrocarbon gas to a heated substrate, excite the gases by heat, discharge or microwave, induce chemical reactions and pile carbon atoms into the diamond structure on the heated substrate.
There are some different CVD methods classified by the differences of exciting the material gas. A hot filament CVD method, a microwave plasma CVD method, a radio-frequency plasma CVD method, a DC plasma jet method and so on have been known for the methods of making a diamond film.